This invention pertains to telephone systems and, more particularly, is concerned with means for alerting a subscriber that a message is waiting.
Institutions such as corporations, hospitals, and hotels operate private automatic branch exchanges (PABX's) which switch calls both between individual phone handsets and with the public system.
A message intended for a subscriber may be manually or electronically recorded at a centralized or satellite message center. In order to alert the subscriber of a message each handset may include a so-called "message waiting" lamp which can be set by the message center to flash. Conceptually, perhaps the simplest way to light a "message waiting" lamp is to run for that purpose a pair of wires to the handset. Most telephone handsets are made with four wires leading out. Two of these wires are not normally connected to the phone system and could be used to carry current to a "message waiting" lamp. This technique is undesirable because of the costs of the extra pair of wires running from the handset and the associated installation costs.
Two wires are used to both transmit and receive. For historic reasons one wire is called the tip line and the other wire is called the ring line. It is preferable to use the tip or ring lines to light a "message waiting" lamp as opposed to providing special wiring. One known arrangement applies an 80 volt DC to the tip line to energize a neon lamp. The DC is actually a 1 Hz square wave to make the lamp flash. A mechanical relay located between the handset and the PABX disconnects the handset's tip line from the PABX when the lamp is energized. Such relays are prone to failure or subtract from the reliability of the system. Furthermore, the 1 Hz DC voltage can generate harmonics or cause interference on the lines.
Alternatively, it is known that lamp voltage may be obtained from a special ring signal from the PABX. This technique requires a close interface between the message center and the PABX.